1. Field of the Invention
The present invention relates to a technique of forming a device with the aid of a superconductive phenomenon, particularly a technique of forming a tunneling Josephson element.
2. Related Art Statement
Prior investigators have sought to form a solid electron device such as a Josephson element with the use of a well-known superconductor such as lead, niobium, an alloy or compound thereof as an electrode material. The Josephson element with the use of these metallic superconductors is near practical use as a result of studies for last ten-odd years.
However, the critical temperature Tc of these superconductors is extremely low (e.g., the highest being an absolute temperature of 23K in case of Nb.sub.3 Ge), so that in order to actuate the element, it is necessary to use liquid helium. The use of liquid helium is a great obstacle for widely using a Josephson element in general. Further, an essential speed of the superconductive device is limited by an energy gap inherent to the superconductor. This energy gap is pointed out to be theoretically proportionate to the critical temperature. Therefore, the low critical temperature limits the speed of the superconductive device.
It has recently become clarified that the noted oxide superconductor such as Y-Ba-Cu-O series is operable at a relatively high temperature (more than 90K) and that the oxide superconductor has a large superconductive gap voltage ten times larger than the prior superconductor. In the above two points, the Josephson element with the use of these materials, as compared with the prior devices, is very advantageous and considered to be industrially valuable in practice. However, when the insulator as a tunnel barrier is deposited, there superconducting oxides are liable to cause a reaction at the interface between the insulator and superconducting oxides. Therefore, the superconducting characteristic on the side of the superconductor is remarkably deteriorated and becomes a large obstacle in manufacture of a device. This is confirmed to be caused due to the structure of the oxide superconductor which is liable to dissociate oxygen and to result in a chemical reaction of atoms in the insulator with oxygen in the superconductor.